Ethylbenzene, C.sub.8 H.sub.10, is a key raw material in the production of styrene and is produced by the ethylation reaction of ethylene, C.sub.2 H.sub.4, and benzene C.sub.6 H.sub.6 in a catalytic environment. Old ethylbenzene production plants, typically built before 1980, used AlCl.sub.3 or BF.sub.3 as acidic catalysts. The newer plants in general have been switching to zeolite-based acidic catalysts. The typical purity of the benzene feed, known as nitration grade benzene, is 99.9 wt %. The typical purity of the ethylene feed would exceed 99.9 mol %.
A significant source of crude benzene is pyrolysis gasoline (C.sub.5 to C.sub.9), which typically contains 55-75 wt % aromatics. Pyrolysis gasoline, produced in naphtha based or heavy liquid based olefin plants, contains 35-55 wt % benzene. About 35% of world's benzene production capacity originates from pyrolysis gasoline. Typically, after pyrolysis gasoline is hydrotreated for saturation of olefins and di-olefins, the pyrolysis gasoline (free of olefins and sulfur compounds) is exported to battery limits for aromatics extraction process. Pure benzene, 99.9 wt %, along with toluene and xylene, is a typical product of aromatic extraction.
Impure benzene, 94-98 wt %, which is a 75-83.degree. C. atmospheric cut, can be recovered from hydrotreated pyrolysis gasoline by a simple fractionation process, as described in U.S. Pat. No. 5,880,320, the disclosure of which is incorporated herein by reference.
Three types of ethylation reactor systems are used for producing ethylbenzene, namely, vapor phase reactor systems, liquid phase reactor systems, and mixed phase reactor systems. In vapor-phase reactor systems, the ethylation reaction of benzene and ethylene is carried out at about 380-420.degree. C. and a pressure of 9-15 kg/cm.sup.2 -g. In most cases, these systems use ethylene feed in pure form as produced in conventional olefin plants. Dilute ethylene streams, about 10-15 vol %, as produced in fluid catalytic cracking (FCC) in petroleum refining, are converted to ethylbenzene using vapor phase reaction. One known facility was designed by Raytheon Engineers & Constructors and is operated by Shell Chemicals at UK. Similar facilities for FCC off-gases were built in China by Sinopec.
Vapor phase reactor systems comprise multiple fixed beds of zeolite catalyst. Ethylene exothermally reacts with benzene to form ethylbenzene, although undesirable chain and side reactions also occur. About 15% of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products. All these chain reaction products are commonly referred as polyethylated benzenes (PEBs). In addition to the ethylation reactions (at times referred to in the industry as alkylation reactions), the formation of xylene isomers as trace products occurs by side reactions. This xylene formation in vapor phase can yield an ethylbenzene product with about 0.05-0.20 wt % of xylenes. The xylenes show up as an impurity in the subsequent styrene product, and are generally considered undesirable.
Additionally, traces of propylene may enter the system with the ethylene feed or are formed by catalytic cracking of non-aromatic impurities that may enter with the benzene feed. The presence of propylene results in the formation of isopropyl benzene, commonly known as cumene, which is very undesirable in the ethylbenzene at concentrations above 150 PPM. The cracking of non-aromatic impurities is accelerated by increasing the ethylation reaction temperature, and thus substantial cracking of non-aromatic impurities to propylene occurs if the ethylation or transalkylation reaction is at temperatures of above 300.degree. C. and in presence of acidic catalyst. This may result in an unacceptable level of cumene in the ethylbenzene product.
In order to minimize the formation of PEBs, a stoichiometric excess of benzene, about 400-900% per pass, is applied, depending on process optimization. The effluent from the ethylation reactor contains 70-85 wt % of unreacted benzene, 12-20 wt % of ethylbenzene product and about 3-4 wt % of PEBs. The PEBs are converted back to ethylbenzene to avoid a yield loss.
The effluent of the ethylation reactor can undergo ethylbenzene product recovery by several multiple fractionation stages. Benzene can be recovered in a benzene recovery column by stripping and can be recycled to the ethylation reactor. Ethylbenzene product can be recovered in an ethylbenzene recovery column. DEB and TEB can be separated from heavier aromatics in a PEB column. The heavy aromatics can be diverted to the fuel oil system.
The DEB and TEB mixture proceeds to a transalkylation reactor system where stoichiometric excess (250-300%) of benzene reacts with DEB and TEB in vapor phase at about 420-450.degree. C. About 60-70% of the PEB is converted to ethylbenzene per pass. The effluent product of transalkylation reactor consists of ethylbenzene, un-reacted benzene and unconverted PEBs. This transalkylated stream undergoes stabilization for light ends removal and is recycled to fractionation in the benzene column. The ultimate conversion of DEB and TEB to ethylbenzene is essentially 100%.
The boiling point of the xylene isomer trace products is very close to that of the ethylbenzene, and thus no practical separation is possible. The ethylbenzene product typically contains 500-2,000 PPM by weight of xylene isomers, as well as 1000-2,000 PPM by weight of benzene.
In recent years the trend in industry has been to shift away from vapor phase reactors to liquid phase reactors. Liquid phase reactors operate about 260-270.degree. C., which is under the critical temperature of benzene, 290.degree. C. One advantage of the liquid phase reactor is the very low formation of xylenes and oligomers. The rate of the ethylation reaction is lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually economically compensates for the negatives associated with the higher catalyst volume. The stoichiometric excess of benzene in liquid phase systems is 150-400%, compared with 400-800% in vapor phase. However, due to the kinetics of the lower ethylation temperatures, resulting from the liquid phase catalyst, the rate of the chain reactions forming PEBs is considerably lower; namely, about 5-8% of the ethylbenzene is converted to PEBs in liquid phase reactions versus the 15-20% converted in vapor phase reactions. Transalkylation reaction, where polyethylated benzene reacts with benzene to form ethylbenzene, can occur in a liquid phase or vapor phase system. The liquid phase reaction temperature would be 230-270.degree. C. The fractionation sequences and product recovery methods for liquid phase reaction systems are similar to those used in connection with vapor phase reactor systems.
In recent years, technology has been developed for the production of ethylbenzene from dilute ethylene streams by a mixed phase reactor. The demonstrated dilute ethylene stream sources are from petroleum refineries, fluid catalytic cracking operation (FCC). ABB Lummus Global and CDTech have developed a mixed phase process. Aside from ethylation reactors, the sequence of the ethylbenzene product recovery and transalkylation is similar to the conventional liquid phase reactor systems.
A potentially alternate source of dilute ethylene is described in a pending U.S. Pat. No. 5,880,320. The dilute ethylene stream is extracted from the demethanizer section of the ethylene plant at about 22-30 kg/cm.sup.2 -g. Dilute gas from ethylene plants may contains 7-25 mol % ethylene, and the bulk of the balance is methane and hydrogen. The propylene content is controlled at the ethylene source to remain below 20 PPM by volume.
The use of a liquid phase reaction system for dilute ethylene streams is not possible. Due to the high methane and hydrogen content in the ethylene stream, the bubble point temperature of the combined mixture of dilute ethylene and benzene is very low, lower than the activity temperature of the ethylation catalyst, and actually below the freezing point of benzene.
The reaction temperature of the mixed phase ethylation reactor is under the dew point of the dilute ethylene benzene mixture, but well above the bubble point. The diluents of the ethylene feed comprise hydrogen, methane and small amounts of ethane, and CO remains essentially in the vapor phase. The benzene in the reactor is split between vapor phase and liquid phase, and the ethylbenzene and PEB reaction products remain essentially in liquid phase.
In the alkylation and transalkylation of aromatic hydrocarbons, zeolite catalysts have been shown to be an adequate substitute for acidic catalysts, such as aluminum chloride (AlCl.sub.3), boron trifluoride (BF.sub.3), liquid and solid phosphoric acid, sulfuric acid and the like. For example, U.S. Pat. No. 2,904,607 shows alkylation of aromatics in the presence of a crystalline aluminosilicate having a uniform pore opening of 6 to 15 angstroms.
U.S. Pat. No. 3,641,177 describes an alkylation process wherein the catalyst has undergone a series of ammonium exchange, calcination and steam treatments. This catalyst would currently be described as an "ultrastable" or "steam-stabilized" zeolite Y catalyst.
U.S. Pat. No. 3,751,504 and 3,751,506 show transalkylation and alkylation over ZSM-5 type catalysts. Use of other medium-pore to large-pore zeolites are taught in U.S. Pat. Nos. 4,016,245 (ZSM-35), 4,046,859 (ZSM-21), 4,070,407 (ZSM-35 and ZSM-38), 4,076,842 (ZSM-23), 4,575,605 (ZSM-23), 4,291,185 (ZSM-12), 4,387,259 (ZSM-12), and 4,393,262 (ZSM-12) and European Patent Application Nos. 7,126 (zeolite omega) and 30,084 (ZSM-4, zeolite beta, ZSM-20, zeolite L).
Liquid phase alkylation is specifically taught using zeolite beta in U.S. Pat. No. 4,891,458 and European Patent Application Nos. 0432814 and 0629549. Novel dealuminized mordenites are described for these types of reactions in U.S. Pat. Nos. 5,015,797 and 4,891,448.
More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions. U.S. Pat. Nos. 4,992,606 (MCM-22), 5,258,565 (MCM-36), 5,371,310 (MCM-49), 5,453,554 (MCM-56), and 5,149,894 (SSZ-25). Additionally Mg APSO-31 is described as an attractive catalyst for cumene manufacture in U.S. Pat. No. 5,434,326.
U.S. Pat. No. 5,176,883 describes an integrated ethylation fractionation in general without diluants for the ethylene feed. U.S. Pat. No. 5,043,506 describes the addition of n-C.sub.5, n-C.sub.6, and i-C.sub.6 as a means for fractionation control in alkylation systems.